Hydraulic control system



Oct. 6, 1956 R. A. WITTREN HYDRAULIC CONTROL SYSTEM Filed JulyV l1, -1955 INVENTOR. R. A. WITTREN HYDRAULIC CONTROL SYSTEM Richard A. Wittren, Cedar Falls, Iowa, assignor to Deere lIVIanufacturing Co., Dubuque, Iowa, a corporation of owa Application July 11, 1955*, Serial No. 521,005

12 Claims. (Cl. 60-52) This invention relates to a hydraulic system particularly useful in the remote control of a hydraulic motor.

The preferred embodiment of the invention finds particular utility in an agracultural tractor and associated implements, wherein the hydraulic pump is located on and driven by the tractor and the motor is relatively remote and associated with a movable implemnt or implement part. It is in general desirable in an arrangement of the character described that extension or retraction of the motor be proportionate to the movement of an initiating actuating member, such as a hand lever. The reason for this is that the arrangement enables the operator to determine the position of the implement on the basis of the position of the hand lever. The success of a system of this character depends upon follow-up means for returning the control valve to neutral after a predetermined range of operation of the motor. In the past, reliance has been placed largely von mechanical linkages for accomplishing the follow-up action but such designs in general suffer from various complications inherent in linkage lengths and lost motion. According to the present invention, these disadvantages are eliminated by the use of a hydraulic follow-up system that utilizes changes in volumetric capacity of a closed hydraulic circuit because of the volumetric difference at opposite sides of the motor piston, which difference ordinarily occurs because of the attachment of a piston rod to one side of the piston. The invention features a control cylinder having a piston movable therein to effect fluid interchange between the control cylinder and a servo cylinder in which is located a piston that is attached to the main control valve. Accordingly, displacement of the control piston initially displaces the servo piston to shift the valve to an active position. The intake of the pump is connected to the servo chamber and a iluid interchange is effected between the return side of the motor and the servo chamber, which interchange is the reverse of the initial interchange so that the valve is automatically returned to neutral. Because the volumetric capacity of the servo chamber limits initial movement of the control piston to a relatively small increment, the invention features the use of biasing means for continual loading of the control pistons so as to continue the iluid interchange and to thereby accomplish a longer stroke in the motor piston.

The actuating means includes a control lever which acts.

on the biasing means and is selectively operative to'vary the loading on the biasing means and thereby to control the extent of movement of the control piston and henceto vary the time interval in which the biasing is effective. Indexing means are used in connection with the control lever so that varying portions of the motor stroke maybe used. The system further has means for re-phasing the4 control and motor pistons.

The foregoing and other important objects and desirable features inherent in and encompassed by the invention will become apparent as a preferred embodiment of the invention is disclosed in detail in the ensuing specification and accompanying drawing, the single ligure of which fjice represents schematically and in section a hydraulic control suitable for the purposes herein before set forth.

The system includes a pump P, a reservoir R, a motor M, a valve V, control means C and actuating means A.

The motor M is herein illustrated as comprising a cylinder 10 and piston 12. A piston rod 14, attached to one side of the piston 12, extends through one end of the cylinder 10 and the presence of the piston rod therefore decreases the area of one side of the piston. Consequently, the piston has opposite sides of unequal areas. In a tractor-implement arrangement, the motor would be mounted on an implement, with the cylinder and piston rod connected respectively to relatively movable parts. The pump, valve, control means, actuating means and reservoir will be on the tractor or, stated otherwise, would be relatively remote from the motor.

The valve V and control means C are here shown as being embodied in a unitary housing H. This housing includes an elongated valve cylinder or chamber 16 in which a valve member 18 is axially shiftable to first and second active positions respectively at opposite sides of a central neutral position. The neutral position is illustrated in the drawings. The valve is of the spool type having three spools 20, 22 and 24 and a pair of intervening reduced portions 26 and 28. The valve cylinder includes a plurality of ports, including a high-pressure port 30, a pair of motor parts 32 and 34 and a pair of low-pressure ports 36 and 38.

The valve includes a secondary housing 40 in which are fluid-actuated check valves 42 and 44, respectively, for controlling the transmission of fluid between the motor ports 32 and 34 and opposite ends of the motor cylinder 10. The check valve 42 normally seats on a Valve seat 46 at the end of a passage or line 48 which leads from the motor port 32. At the opposite side of the check valve 42, the housing 40 includes a check valve chamber 50 from which a line 52 runs to the motor cylinder 10 at the small-area side of the motor piston 12. The pump has its discharge side 54 connected by a line 56 to the high-pressure valve port 30 and this line has a branch 58 exposed t0 a check valve piston 60 that is carried slidably in the secondary housing 40 in coaxial relationship to the check valve 42. The check valve has a stem 62 received in the check valve piston 60. A spring 64 is normally effective to seat the check valve 42 and to keep the check valve piston 60 at the bottom of a check valve piston chamber 66.

The other check valve 44 is symmetrically arranged in the housing 40, being normally seated by a spring 68 on a valve seat 70 which separates a check valve chamber 72 from a line 74 that leads to the other motor port 34. The check valve 44 has a stem 76 received by a check Valve piston 78 that is slidably carried in a lower check valve piston chamber 80 and one end of the piston 78 is exposed to the high-pressure line 56 via a branch 82. The check valve chamber 72 is connected by a line 84 to the motor cylinder 10 at the large-area side of the motor piston 12.

The valve member 18 is normally neutrally positioned by centering means N and, for this purpose, the valve member has at one end thereof an extension rod 86 to which is attached a cam 88. A central notch 90 in the cam receives one end of a plunger 92 which is biased to cam-engaging position by a spring 94. The plunger eX- tends into a relief valve housing 96 in which is carried a relief Valve piston 98 normally seated at 100 by a relief Valve spring 102. The relief valve seat 100 is exposed to the discharge side of the pump by a fluid line 104.

The low-pressure ports 36 and 38 of the valve V are connected in common to a return line 106 which is connected by a line 108 to the pump intake at 110. A relief return line 106.

In the neutral position of the valve member 18, fluid is circulated by the pump P at no appreciable pressure, since the high-pressure port 39 is connected to the lowpressure ports 56 and 38 and these are in turn connected to the return line 1636 which leads via 108 to the pump intake 116. The relief valve 9S is seated at 166 and both check valves 42 and 44 are closed. Thus, the piston 12 in the motor M is hydraulically locked. When the valve member 1S is shifted upwardly (as by means to be described), the central spool 22 separates the high-pressure port 36 from the motor port 32, but the reduced valve portion 26 interconnects the motor port 32 and the low-pressure port 36. Simultaneously, the lowermost spool 24 cuts oit the low-pressure or return port 33, and the reduced portion 28 on the valve member 18 interconnects the high-pressure port 36 and the motor port 34. The initial rise in pressure in the pump discharge line 56 and branches S and 82 moves the check valve pistons 60 and 78 to open the check valves 42 and 44, respectively. Fluid under'pressure is thus transmitted from the motor port 34 to the large-area side of the motor piston 12 via the conduit made up by the line 74, the check valve chamber 72 and the line 84. Fluid returning from the small-area side of the motor piston 12 is transmitted by the conduit comprising the line 52, the check valve chamber Si) and the line 4S. The foregoing describes the functioning of the circuit with the valve member 18 in one of its two active positions.

rlhe other active position of the valve member is achieved when the valve member is moved downwardly, which results in retraction of the moto-r M, because iluid under pressure is supplied to the motor at the small-area side of the motor piston 12 and is returned by the motor from the large-area of the motor piston. In brief, when the valve member 16 is shifted downwardly, the central spool 22 separates the high-pressure port 3i) from the motor port 34 but the reduced valve portion 28 connects the motor port 34 with the low-pressure port 38. Simultaneously, the upper spool portion 20 blocks the iowpressure port 36 but the reduced valve portion 26 inte.- connects the high-pressure port 36 and the upper motor port 32. Again, pressure rise in the pump discharge line 56 operates through the branches 58 and 82 to move the check valve pistons 6B and 78 for opening the check valves 4 2 and 44, respectively. Fluid under pressure is therefore transmitted to the upper end of the motor M (to the small-area side of the piston 12) via the conduit comprising the line 4S, the check valve chamber Si) and the line 52. Fluid returning from the large-area side of the motor piston 12 is carried by the conduit made up of the line S4, the lower check valve chamber 72 and the line 74.

In each of the active positions of the valve member 18, the plunger 92 is forced to the left against the spring 94, since the end of the plunger rides up the ramps that define the notch 96 in the valve cam 88. The design of the cam is such that if the force holding the valve in its active position is relieved, the spring 92 will centerv the valve in its neutral position. If the valve member is forcibly but releasably held in one or the other of its active positions, it will be forcibly returned to neutral by the relief valve piston 93 in the event of an abnormal rise in pressure in the system, which increase in pressure will be transmitted through the line 104 to force the relief valve piston 93 to the right, causing it to engage the left hand end of the plunger 92. Excess pressure will be returned to the return line 166 via the line 112.

According to the present invention, axial shifting of the valve member 1S is eected by the actuating means A and the control means C. As an instrumental part of this phase of the invention, the valve cylinder 16 extends axially beyond both ends of the valve member 1S so as to aiford upper and lower servo chambers 114' and 116. The upper radial end of the valve member 18 aords a servo piston 118 and another servo piston 120 is afforded by the radial lower end of the valve member. AS

will be seen, iluid supplied selectively to one or the other of the servo chambers will result in axial displacement of the valve member in the selected direction.

Initial displacement of the valve member 18 is brought about by selective iiuid interchange between the control means and the servo chambers. For this purpose, the combined control means and valve housing 16 has a control chamber or cylinder 122 provided with opposite ends which are connected by fluid lines 124 and 126, respectively, with the servo chambers 114 and 116. A control piston 123, having a suitable annular seal 13D, is axially movable in the control cylinder 122 and has a piston rod 132 extending through the upper end of the cylinder. The junction of the upper fluid line 124 with the upper end of the control cylinder 122 includes a valve seat 134 with which is cooperative a stop valve 136 on the upper side of the control piston 12S. The junction of the lower fiuid line 126 with the lower end of the control cylinder 122 includes a valve seat 138 with which a stop valve 140 on the control piston 128 cooperates. The details of operation of the stop valves 136 and 140 will be described below.

A pump line 142 interconnects the lower end of the control cylinder or chamber 122 with the pump intake via the pump intake line 10S. The junction of the pump line 142 with the lower end of the control cylinder is effected at a port 144 which is enlarged to cooperate with a relieved or recessed portion such as an upper annular bevel 146 on the control piston 128. Briefly, when the control piston 128 is at the lowermost limit in the control cylinder 122, the stop valve 141) is received by the valve seat-138 and the relief or recess at 146 permits communication between the upper portion of the control cylinder and the duid line 142. ln other words, the relationship at 146 and 144 establishes means for by-passing the piston 128 when the piston is in its lowermost position. f

A similar relationship exists at the upper end of the control cylinder, which is connected by a reservoir line 148 with the reservoir R. The junction of thevreservoir line with the upper end of the control cylinder or chamber includes a port 150 which operates in conjunction with a second relief or recess in the form of a lower annular bevel 152 on the-piston 12S to establish a by-pass when the piston-is at the upper end of the control cylinder.

The reservoir line 148 includes a pressure-responsive valve 154 which is combined with a make-up valve 156. A valve seat 158 in the valve 154 is normally closed bythe malre-up valve 156, which is in the form of a ballloaded by a relatively light spring 160. A relativelyheavier spring 162 normally seats the pressure-responsivev valve 154 at 164 so as to disconnect the reservoir line 148 from uid in the reservoir.

The purpose of the pressure-responsive valve 154 is to insure filling of the upper servo chamber 114 by upward movement of the control piston 12S-beforey excess fluid is Vdiverted to the reservoir. The purpose of the make-up valve is to allow reservoir uid to enter the upperV end of the control cylinder via the reservoir line 148 when the control piston 128 is moved downwardly.

The `actuating means A, which has been previously referred to as being mounted on a tracto; in remote relationship to `the motor M, includes a iixed support S. This support includes a rockshaft'166 on which is mounted an arm 168 provided with a pin and slot connectionv 1704 to the upper end of the control piston rod 132.` The supportV S' further'includes anv arcuate memberYor-seetor 172 Vwhich is formed with an arcuate slot 174. Front and rear indexing stops 176 and` 178, which may be of' anyknown type, are selectively positionable alongA the slot 17'4 to adjustably determine the angular range of movement of an actuating member in the form of a hand lever 181) that is pivotally mounted on the rockshaft 166. A torsion spring 184 is carried by the rockshaf-t 166i and has one leg 186 bent aroundk the front of a pin 182 on the arm 168` and engaging the front side of a second pin 18.81 onv the lever 180. The spring has a secondA leg,`

190 in engagement with a pin 192 on the arm 4168. A second and reversely positioned torsion spring 194 has a pair of legs 196 and 198 reversely arranged relative to the legs 186 and 190 of the torsion spring 184 so as to reversely cooperate with the pins 182, 188 and 192. When the hand lever 180 is neutrally positioned relative to the actuating arm 168, the springs 184 and 194 are pre-loaded between the pins 182 and 192 on the arm 168 and hold the lever 186 in position by means of the pin 188. Movement of the hand lever 180 in a clockwise direction increases the loading on -the spring 184 and thus causes the arm 168 to exert a downward force on the piston rod 132. Counterclockwise rocking of the hand lever 180 loads the other spring 194 and therefore causes the actuating arm 168 to exert an upward force on the control piston rod 132. Thus, the springs 184 and 194 constitute two-way biasing means under control of the hand lever 180 for effecting selective movement of the control piston 128. Any selected position of the hand lever 180, within the limits of the indexing stops 176 and 178, can be retained by any conventional means cooperative between the lever 180 and the sector 172. Known means include frictional devices of various types and it will be assumed that any such device can be used here. Accordingly, the hand lever 180 is settable in any one of several positions through a relatively wide angular range and movement of the control piston 128 will therefore be in proportion to angular movement of the hand lever. The purpose of the biasing means 184-194 is to permit overtravel of the hand lever 180 relative lto the control piston 128, since, as will presently appear, initial movement of the control piston is limited by the volumetric capacity of whichever of the servo chambers 114 or 116 is affected at the time.

Operation The drawing shows all parts in their respective neutral positions with the control piston 128 midway between the upper and lower ends of the control cylinder and the motor piston 12 midway between 4the ends of the motor cylinder 10. The motor cylinder is not in scale with the control cylinder but the principles of the invention will nevertheless be understood from the following.

As previously described, the valve member 18 is neutrally positioned by the centering means N and the central position, as well as other positions, of the control piston 128 is established by the position of arm 168. The closed check valves 42 and 44 hydraulically lock the motor piston 12 in its position. A closed hydraulic circuit may be considered to exist among the remote cylinder, the pump, the check valves, the control valve and the underside of the control piston. This circuit does not include the volumes above the control valve and above the control piston 128 and valve means 154-156.

If it is desired to retract the motor M, the hand lever 180 is moved in a counterclockwise direction. Let it be assumed that the movement is through the angular range permitted by the stop 178. The biasing spring 194 acts as a resilient connection between the lever 180 and the arm 168 so that the arm likewise moves in a counterclockwise direction and exerts an upward or lifting force on the control piston 128. As the control piston moves upwardly, it displaces uid from the control chamber to the upper servo chamber 114 via the upper iiuid line 124 and the iiuid interchange thus effected causes the valve member 18 to move downwardly. The change in volumetric capacity below -the control piston 128 accommodates the displacement of fluid from the lower servo chamber 116 to the lower end of the control cylinder via the lower fluid line 126. The pressure-responsive valve 154 prevents the exhaust of fluid to the reservoir via the reservoir line 148, at least until the servo chamber 114 is filled. Moreover, the volumetric capacity of the servo chamber 114 will limit initial movement of the control piston 128 to a relatively small increment, thereby permitting an angular counterclockwise range of the arm 168 to a proportionally small increment. However, counterclockwise movement of the hand lever 180 to the position determined by the stop 178 is possible through an angular range and at a rate materially greater than the initial angular increment and rate permitted to the arm 168. Therefore, the hand lever 180 has overtravel as respects the arm 168, which overtravel is permitted by the torsion spring 194. The overtravel of the hand lever will load the torsion spring 194 so that the biasing means continues to exert an upward force on the control piston 128 and this force will be sufficient to overcome the detent action of the piston seal 130 and to hold the valve 18 down against the centering action of the spring 94.

When the valve member 18 is shifted downwardly to the active position just described, the high pressure port 36 is connected to the motor conduit established by 32, 48, 5t) and 52, and uid is thus supplied under pressure to the small-area side of the motor piston 12. Fluid expelled from the large-area side of the motor piston 12 returns to the pump intake via the conduit established by 84, 72, 74, 38 and 106. Because of the dilerence in areas at opposite sides of the motor piston 12, caused by the presence of the piston rod 14, the volume of returning fluid will exceed that of the high pressure uid. The volumetric excess must be diverted from the pump intake, since the pump intake must equal the pump output. This excess is diverted to the control chamber below the piston 128 via the pump line 142 and, as long as the biasing spring 194 continues to cause the control piston 128 to move upwardly, the lower part of the control cylinder 122 will accommodate the volumetric excess. Fliud above the control `piston 128 will open the pressure-responsive valve 154 for diversion to the reservoir R. When the angular relationship between the hand lever 180 and the actuating arm 168 again reaches the point at which the force exerted by the biasing means 184-194 becomes zero, the control piston 128 will cease its upward movement. The lower part of the control cylinder 122 can no longer accommodate the volumetricv excess and this excess is now diverted to the lower servo chamber 116 via the lower fluid line 126, and the resulting volumetric increase in the servo chamber acts on the servo piston to return the valve 18 to its neutral position, whereupon movement of the motor piston 12 ceases. As the valve member 18 is returned to neutral by the servo action at 116, fluid is displaced from the upper servo chamber 114 to the reservoir via the reservoir line 148 and pressure-responsive valve 154.

The foregoing, stated briefly, means that as the control piston is moved at some constant rate, as by the hand-lever-adjusted torsion spring 194, the remote cylinder will retract at a corresponding rate as long as the control valve remains out of its neutral position. As

will be seen, shifting of the valve toward its action position may be only partial, because the amount of displacement thereof. is proportional to the rate of movement of the control piston. Therefore, the rate of the movement of the control piston determines the rate of movement of the motor piston. Stated otherwise, if the control piston is moved rapidly to displace the valve member its maximum amount, the rate of movement of the motor piston will be rapid. The rate will be proportional to the angular rate of `the hand lever vup to the point where this ratio exceeds the pump output after which the motor rate will be constant and equal to the pump output regardless of how fast the hand lever is moved.

It is also possible to consider that the rate maybe varied.

by the extent of loading on the biasing spring 194 by the amount of overtravel imposed on the hand lever'180.

In the event that the control piston 128'reaches its upper limit before the motor piston 12 reaches its lower aware@ limit, the stop valve 136 on the control piston will seat in the valve seat 134 at the upper end of the control cylinder, thereby blocking the fluid line 124 to the servo chamber 114. Hence, excess fluid, previously accommodated by the lower servo chamber 116 after stopping of the control piston 128, cannot be accepted by the lower servo chamber. However, when the control piston 128 moves to its upper limit so as to close the valve at 134-136, it simultaneously opens the by-pass valve 152-150 and the volumetric excess in the control cylinder 122 thus by-passes the piston 128 and escapes to the reservoir through the pressure-responsive valve 154. The motor piston 12 can then extend to its limit and, when it reaches this point, the pressure in the line 104 rises sufciently to unseat the pressure relief valve piston 98 for returning the excess pressure to the return line via the line 112. At the same time, the piston 98 engages the plunger 92 and automatically causes return of the valve member 18 to its neutral position. In a preferred construction, maximum movement of the control piston 128 will be co-extensive with the maximum travel of the motor piston 12.

Extension of the motor M by movement of the piston 12 upwardly from the central position illustrated in the drawings is achieved by clockwise rocking of the lever 180, which acts through the biasing spring 184 to cause the arm 168 to impose a downward force on the control piston 128. As the control piston 128 moves downwardly, at a rate depending upon the rate of movement of the hand lever 180, it displaces fluid to the servo chamber 116 via the fluid line 126. As the volume of the servo chamber is thus increased, the valve member 18 is shifted proportionately upwardly to its other active position, effecting an interconnection of the high pressure line S6 to the large-area side of the motor piston 12 via the conduit established at 74, 72 and 84. Fluid returning from the small-area side of the motor piston 12 travels through the conduit established at 52, 50, 48 and 32 and enters the return line 106 which is in communication with the pump intake 110 via the line 108. Since there will exist a volumetric deliciency in fluid returned from the small-area side of the motor piston 12, and since the pump intake must equal the pump output, the diiiciency must be made up elsewhere. pump line 142, via the line 188, connects the pump intake 110 to the control cylinder 122 and thence via the line 126 to the servo chamber 116, the deciency will be made up from the servo chamber unless the biasing means 184 continues to exert a downward force on the control piston 128. In the event of the existence of such biasing force, the control piston 128 will continue to move downwardly and the volumetric deficiency will be supplied to the pump intake from the control chamber, thus leaving the valve member in its displaced or 7 active position. However, as soon as the biasing force becomes zero, according to the angular increment of movement given to the control lever 180, the deficiency will be made up from the servo chamber 116 and the valve member will return to neutral. Again, the rate of movement of the control piston 128 determines the rate of movement of the motor piston 12. In short, when the volume of piston rod extended from the motor cylinder 10 equals the volume of change introduced by the increment of movement of the control piston 128, the valve 18 will return to its neutral position. Therefore, the relative distances traveled by the motor piston and rod and the control piston 128 are proportional to their respective cross sectional areas. lf these areas are equal, the control piston stroke will equal the remote cylinder piston stroke. The area of the remote cylinder piston is immaterial.

In the event that the load imposed on the biasing spring 184 is sufficient to urge the control piston 128 to its maximum limit of downward travel, and this limit is Because the attained before `the motor piston 12 reaches the u ppetj end of the motor cylinder 10, the stop valve 148 on the bottom of the control piston 128 will seat at 138 andvvill position to cause extension of the motor M, the volumey in the servo chamber now cannot change, because of the valve closure at 13S-140. Fluid necessary to the continued extension of the motor M is derived from the control cylinder 122 above the piston 128, which is elccted because of the by-pass established at 146-144. It will be understood that in either of the by-passing positions of the piston 128, the effectiveness of the annular seal 130 as a seal is temporarily destroyed by the enlarged ports 144 `or 15G and the reliefs at 146 or 152, respectively. Again, the maximum limit of the motor piston 112 is accompanied by an abnormal pressure rise in the line 84, resulting in actuation of the relief valve 98 and plunger 92, which again restores the valve member 18 to its neutral position. In short the by-passing function results in re-phasing of the control and motor pistons.

In the practical use of the system in an agricultural tractor-implement organization, maximum extension and retraction of the motor M is permitted by placing the indexing stops 176 and 178 respectively at their maximum front and rear positions in the sector slot 174. A shorter stroke in either direction may be obtained by selecting a new position for one or the other or both of the indexing stops, which will therefore limit the angular range of the hand lever 180 and will accordingly limit the amount of loading that can be placed on the biasing means in either direction. Provision for over-riding the indexing stops by the hand lever may be readily incorporated in the design.

As illustrated, the check valve arrangement at 42 and 44 is shown in a separate housing but this detail has no significance so far as concerns the invention. Likewise, the ends of the valve member 18 are utilized as the servo pistons 118 and 120, but it is obvious that separate servo pistons could be used. Other modifications and alterations will readily occur to those versed in the art, all of which may be achieved without depart ing from the spirit and scope of the invention.

What is claimed is:

l. A hydraulic control system, comprising: a motor having a piston provided with opposite sides of unequal areas; a pump; a normally neutrally positioned valve member shiftable selectively to lirst and second active positions for respectively connecting the pump discharge and pump intake respectively to the large-area and smallarea ysides of the motor piston and vice versa; servo means for shifting the valve member to and from its active positions including iirst and second servo chambers respectively having first and second servo pistons connected to the valve member and selectively fluid-displaceable to shift the valve member; control means including a control chamber having rst and second opposite ends connected respectively by first and second uid lines to the first and second servo chambers, said control means further including a control piston movable in a rst direction to add and withdraw liuid respectively to and from the first and second servo chambers for incurring the first active position 'of the valve member, and said control piston being movable in a second, opposite direction to add and withdraw fluid respectively to and rom the second and first servo chambers for incurring the Second active position `of the valve member; and a pump line interconnecting the first end of the control chamber and the pump intake and serving, when the lirst active position of the valve is incurred by the control piston and servo means, to withdraw fluid from the fir-st servo chamber to supplement the volumetric deiciency returned to the pump intake from the small-V area side of the motor piston and to thereby incur the' neutral position of the valve member, said pump line serving, when the second active position of the valve member is incurred by the control piston and servo means, to add fiuid to the first servo chamber from-the volumetric excess returned to the pump intake from the large-area side of the motor piston and to thereby incur the neutral position of the valve member.

2. The invention defined in claim 1, including: a reservoir; a reservoir line leading from the reservoir to the second end of the control chamber; and pressureresponsive valve means in the reservoir line operative to prevent reverse fiow `of fiuid from the control chamber to the reservoir until the second servo chamber is first filled from the control chamber.

3. The invention defined in claim 1, including: a stop valve operative in response to predetermined first-direction movement of the control piston to block the first fiuid line and thereby to i-solate the first servo chamber from both the pump intake and the control chamber; and by-pass means operative in conjunction with the stop valve for by-passing the control piston and thereby interconnecting the pump line and the control chamber at the Second-end side of the control piston.

4. The invention defined in claim 3, including a reservoir; a reservoir line leading from the reservoir tothe second end of' the control chamber; and pressure-responsive valve means in the reservoir line operative to prevent reverse fiow of fiuid from the control chamber to the reservoir until the second servo chamber is first filled from the control chamber; an additional stop valve operative in response to predetermined second-direction movement of the control piston to block the second fiuid line and thereby to isolate the second servo chamber from both the control chamber and the reservoir line; and additional by-pass means operative in conjunction with the additional stop valve for by-passing the control piston and thereby interconnecting the reservoir line and the control chamber at the first-end side of the control piston.

5. The invention defined in claim 3, in which: the, control chamber is a cylinder having at its first end a radial wall centrally apertured to afford a valve seat opening to the first fiuid line; and the stop valve is carried by and centrally of the piston for cooperation with said seat.

6. The invention defined in claim 3, in which: the bypass means comprises a recess at one edge of the control piston adapted to register with the pump line junction to the control chamber when the control piston has attained said predetermined first-direction movement.

7. The invention defined in claim 1, in which: the valve member is cylindrical and is carried in a valve cylinder having first and second ends extending axially beyond the valve member to comprise, respectively, the first and second servo chambers; and the first and second servo pistons are first and second piston areas respectively on the radial ends of the valve member.

8. The invention defined in claim 1, including: releasable means cooperative uth the control piston for holding said control piston at selected pisitions in its range of movement; and control means connected to and for forcibly moving the control piston to overcome the effect of the releasable means.

9. A hydraulic control system, comprising: a motor having a piston provided with opposite sides of unequal areas; a pump; a normally neutrally positioned valve member shiftable selectively to an active position for respectively connecting the pump discharge and pump intake respectively to the large-area side of the motor piston; servo means for shifting the valve member t`o and from its active position, including a servo chamber having a servo piston connected to the valve member and fiuid-displaceable to shift the valve member; control means including a control chamber having one end connected by a fiuid line to the servo chamber, said control means further including a control piston movable toward said one end to add fluid to the servo chamber for incurring the active position of the valve member; and a pump line interconnecting said end of the control chami ber and the pump intake and serving, when the active position of the valve is incurred by the control piston and servo means, to withdraw fluid from the servo chamber to supplement the volumetric deficiency returned to the pump intake from the small-area side of the motor piston and to thereby incur the neutral position of the valve member.

10. The invention defined in claim 9, including: actuating means for moving the control piston through an initial increment limited by the volumetric capacity of the servo chamber, and said actuating means including biasing means acting on the control piston and effective to continue movement of the control piston beyond said increment for displacing additional fiuid from the control chamber to the servo chamber as fiuid is Withdrawn from said servo chamber by the pump intake.

l1. The invention defined in claim 10, in which: the actuating means includes a selectively settable actuating member acting through the biasing means to move the control piston initially and having a range of movement greater than said initial increment for varying the loading on the biasing means.

12. A hydraulic control system, comprising: a motor having a piston provided with opposite sides of unequal areas; a plump; a normally neutrally positioned valve member shiftable selectively to an active position for connecting the pump discharge and pump intake to the motor respectively at opposite sides of the motor piston; servo means for shifting the valve member to its active position, including a servo chamber having a piston connected to the valve member and fluid-displaceable to shift said valve member; control means including a control chamber connected at one end to the servo chamber by a fluid line and having a control piston movable therein to effect fiuid interchange between the control and servo chambers for displacing the servo piston to incur the active position of the valve member; and a pump line interconnecting the servo chamber and the pump intake to effect fiuid interchange between the pump intake and the servo chamber according to the volumetric difference in motor fluid returned to the pump intake by operation of the motor when the valve member is actively positioned, said motor to-servo fiuid interchange being opposite to the control chamber to servo fiuid interchange so as to effect the return of the valve member to neutral.

References Cited in the file of this patent UNITED STATES PATENTS 752,491 Warren Feb. 16, 1904 1,955,154 Temple Apr. 17, 1934 2,637,303 Cintron May 5, 1953 

